In medium saucepan, heat oil over medium heat. Add onion, and cook, stirring frequently, until softened, 5 minutes. Add zucchini and garlic, and cook, stirring frequently, until softened, 5 minutes. Add 2 cups water, and bring to a boil. Slowly stir in polenta. Cook, stirring constantly, until thickened, about 5 minutes. Stir in tomato, cheese and parsley.